Many devices such as computers cannot tolerate even short interruptions in the supply of electricity or even slight deviations from rigid quality specifications for power. In addition, many manufacturing processes such as semiconductor fabrication have similar requirements. Power interruptions or poor quality power can result in loss of data, computers going off-line, damage to hardware, loss of in-process product, delayed product deliveries and other problems.
One solution has been to use generator sets that provide precise power to the critical loads and only to the critical loads. Thus, the power that has to be precise is isolated from utility problems such as switching transients, and lightning strikes as well as from in-house problems such as power surges or voltage dips caused by motor starting inrush. Generally this has meant the installation of what has been referred to as an “n+1” system. In other words, one more generator set than is necessary to meet the maximum load is installed. Thus, if a generator set fails, the remaining generator set(s) can still handle the load.
A major deficiency in “n+1” systems is the increase in capital cost due to the redundant generator set. Thus, if two generator sets can handle the load, three must be installed for roughly a 50% cost increase. A worse case occurs if one generator set can handle the load. Then the addition of a redundant generator set essentially doubles the cost of the system.
A second deficiency of an “n+1” system is that the generator sets do not operate at their full rating as they must always be prepared to increase power to handle the required load when a generator set fails. Indeed, if one generator set can handle the load and two are installed, the two generator sets must operate at no more than half of their rating if they are to be able to pick up the full load when a generator set fails. With conventional single shaft, gas turbine-driven generator sets (turbogenerators), this has meant a dramatic decrease in fuel efficiency. Correspondingly, small turbogenerators or microturbines must operate at much higher speeds than is optimum for efficiency if they are going to be able to assume the increased load when a generator set fails.
U.S. Pat. No. 4,059,770, issued on Nov. 22, 1977 to Robin Mackay entitled “Uninterruptible Electric Power Supply” describes a concept which uses a prime mover to drive two synchronous generators simultaneously through an overrunning clutch, and is incorporated herein by reference. The first or critical generator would be connected to the critical load and provide precise power to it while always matching exactly the demand of the critical load. The second or non-critical generator would be paralleled with the utility and assist the utility in providing power to the non-critical loads. It would provide a varying amount of power to the non-critical loads such that the total load of both generators would be fixed and match the rating of the prime mover. Thus, if the critical generator experienced a rapid change in load through a change in the load on the critical buss or a failure of a paralleled generator set, the prime mover would not see a step load. It would continue to drive the generators at a fixed speed maintaining a constant frequency.
Correspondingly, if the prime mover in the concept of U.S. Pat. No. 4,059,770 failed, the generator that is connected to the utility grid would instantly become a motor and drive the generator that is feeding the critical loads. The prime mover would instantly disconnect itself from the generators through the action of an overrunning clutch.
There are several advantages to this system over an “n+1” system. First, the redundant generator set is eliminated reducing first cost. Second, the prime mover operates at its full rating which increases its efficiency. Third, because the generator set operates at its full rating, more power is generated reducing the capital cost per kWh element in the cost of producing electricity. Fourth, because the output of the prime mover does not change as the critical loads change, there is no problem with positive or negative step loads.
While the concept of U.S. Pat. No. 4,059,770 has these advantages with respect to turbogenerators generally, small turbogenerators or microturbines present a different problem statement. The vast majority of microturbines in the field today consist of modest sized gas turbines that drive generators that operate at the same speed as the gas turbine. These very high-speed generators operate in the range of 96,000 rpm to produce power at very high frequencies, typically 1600 Hz. As this frequency is too high for most applications, a power conditioning system is needed. The generator output is rectified to direct current and then inverted to conventional frequencies such as 50 Hz or 60 Hz.
For precise power using microturbines, an “n+1” system has traditionally been used with the disadvantages discussed above. Although the concept of driving two generators simultaneously, as proposed in U.S. Pat. No. 4,059,770, is theoretically possible, it is totally impractical because of the very high speeds.
An alternate concept more specifically directed to microturbines is described in U.S. Pat. No. 6,031,294, issued on Feb. 29, 2000 to Everett R. Geis, Brian W. Peticolas and Joel B. Wachnov entitled “Turbogenerator/Motor Control System with Ancillary Energy Storage/Discharge” and incorporated herein by reference. In that patent, an alternate source of power such as batteries can be used either to provide additional power to the direct current buss during increasing loads or to absorb power during decreasing loads.
Although technically sound, this concept increases the cost of the system as well as the space required, as the most practical source of additional power is batteries. Batteries are expensive, bulky, and require both regular maintenance and regular replacement. In addition, the gas turbines still operate at reduced load, as there must be margin to handle increasing loads or potential overloads. This impacts the efficiency as well as reducing the total amount of power that the gas turbine could produce thus increasing the capital cost per kWh generated. Most important, in the event of a gas turbine failure or a fuel supply failure, the system would go down as soon as the energy storage device (battery) was exhausted.